Measurement of sub threshold resonance contributions to fusion reactions: the case of the C-13(alpha, n)O-16 astrophysical neutron source

The C-13(alpha, n)O-16 reaction is the neutron source for the main component of the s-process. It is is active inside the helium-burning shell of asymptotic giant branch stars, at temperatures less than or similar to 10(8) K. In this temperature region, corresponding to an energy interval of 140 - 230 keV, the C-13(alpha, n)O-16 cross section is dominated by the -3 keV sub-threshold resonance due to the 6.356 MeV level in O-17. Direct measurements could not establish its contribution owing to the Coulomb barrier between interacting nuclei, strongly reducing the cross section at astrophysical energies Similarly, indirect measurements and extrapolations yielded inconsistent results, calling for further investigations. The Trojan Horse Method was applied to the C-13(Li-6, n(16)O)d quasi-free reaction to access the low as well as the negative energy region of the C-13(alpha, n)O-16 reaction. By using the generalized R-matrix approach, the asymptotic normalization coefficient ((C) over tilde (17O(1/2+))(alpha 13C))(2) of the 6.356 MeV level was deduced. For the first time, the Trojan Horse Method and the asymptotic normalization coefficient were used in synergy. Our indirect approach lead to ((C) over tilde (17O(1/2+))(alpha 13C))(2) = 7.7(-1.5)(+1.6) fm(-1), slightly larger than the values in the literature, determining a C-13(alpha, n)O-16 reaction rate slightly larger than the one in the literature at temperatures lower than 10(8) K, with enhanced accuracy.